This invention relates to the severing and sealing of thermoplastic materials, and more particularly to the use of a heating element operating at a temperature above about 1050.degree. F. to simultaneously sever and seal edge portions of layers of thermoplastic material used to form bags or the like.
In the process of manufacturing thermoplastic bags or other containers which may include a thickened closure or other thickened sections of thermoplastic material, the bag stock is typically supplied in the form of a continuous web. In forming individual bags, portions of the thermoplastic material are severed from the web. These severed areas become the side seams for the bag and are typically sealed at the same time the material is severed from the web.
To accomplish this severing and sealing operation, a "cutter-sealer" element is used which is typically in the form of a metal alloy wire of circular or elliptical cross-section. To cut through and seal both the main body and thickened portions of the thermoplastic material, such as a polyethylene resin, the wire is heated to a temperature above about 600 degrees F., typically 800 to 900 degrees F., which is above the melting temperature of the material.
A significant problem with such prior art cutter-sealer elements was their relatively short lifespan. Because of the elevated temperatures of operation and stresses involved with severing the thermoplastic materials, prior cutter-sealer elements suffered from oxidation of the metal, creep and fatigue, and stress ruptures. Accordingly, operating temperatures were attempted to be kept low to increase element life. The down time of the equipment used for plastic bag production due to cutter-sealer element failure was significant, and frequent replacement of the cutter-sealer elements was expensive.
Another problem with such elements was that degraded polymer, created by the contact of the hot element with the thermoplastic resin, would result in the formation of waxy substances which would build up on the cutting and sealing equipment, thereby reducing its efficiency. Frequent cleaning of the equipment was required, with concomitant down time for the bag production operation.
Contact of the themoplastic resin with the hot element would also produce gaseous polymer degradation products during the severing and sealing operation. Some of those gaseous products would become entrapped on the surface or in the bags which were formed. The presence of such gaseous degradation products produced noticeable odors, as well as producing off-flavor problems for foods later stored within the bags.
More recently, there have been attempts to address these problems. For example, Tumminia, U.S. Pat. No. 4,396,449, addressed the wax buildup and fatigue problems on the sealing equipment by designing a cutter-sealer element profile having a much greater height than width. The Tumminia cutter-sealer elements were fabricated from nickel and/or chromium alloys such as Inconel 625 or Chromel A.
However, problems with such cutter-sealer elements do remain. Accordingly, the need still exists in the art for a cutting and sealing element and process which provides a long cutter element life, avoids wax buildup on the sealing equipment, and alleviates taste and odor problems.